This invention relates to computed tomography equipment and the like and specifically to an x-ray detector for computed tomography and for determining the z-axis position of a fan beam of x-rays employed in such systems.
Computed tomography (CT) systems, as are known in the art, typically include an x-ray source collimated to form a fan beam, the fan beam extending generally along a fan beam plane and directed through an object to be imaged. After passing through the imaged object, the fan beam is received by an x-ray detector array extending along the fan beam plane. The x-ray source and detector array are rotated together on a gantry within an imaging plane, generally parallel to the fan beam plane, around the image object.
The axis of rotation of the gantry will be designated as the z-axis of the Cartesian coordinate system and the fan beam plane and imaging plane will be generally parallel to the x-y plane of the coordinate system.
The detector array is comprised of detector cells each of which measures the intensity of transmitted radiation along a ray from the x-ray source to that particular detector cell. At each gantry angle, a projection is acquired comprised of intensity signals from each of the detector cells. The gantry is then rotated to a new gantry angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
Each tomographic projection set is stored in numerical form for later computer processing to "reconstruct" a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT or may be converted to a film record by means of a computer driven camera.
Ideally, the fan beam plane will strike the center line of the detector array. In practice, however, the fan beam plane may be displaced from the center line because of two effects. The first effect is the thermal expansion of the x-ray tube's anode and its support. The surface temperature of the tube's anode may rise as high as 2000.degree. C. and the anode supporting structure may rise to 400.degree. C. or more. This heating and the resulting expansion of tube's anode and its support causes a shift the focal spot of the tube which moves the point from which the x-rays emanate. The shifting of the focal spot causes a corresponding shift in the fan beam plane.
The second effect is the mechanical deflection of the gantry and anode support as the gantry rotates. This deforming stress results from the changing angle of gravitational acceleration and the changing magnitude of centripetal acceleration as a function of the rotational velocity of the gantry, acting both on the gantry and anode.
Displacement of the fan beam plane from the center line of the detector array is a problem because it causes variations in detector signal that are "exogenous" or unrelated to the internal structure of the imaged object. Generally each detector cell's sensitivity to x-rays will be a function of the z-axis position of the fan beam along the surface of that cell, that is, the detector cells exhibit a "z-axis sensitivity". This z-axis sensitivity, combined with motion of the fan beam plane on the detectors, produces the undesired variations in the strength of the detector signal. Such exogenous variations in the detector signals produce undesirable ring like artifacts in the reconstructed image.
Compounding the problem of correcting for z-axis sensitivity is the fact that the z-axis sensitivity generally differs among different detector cells in the detector array. This difference will be termed "intercell sensitivity variation".
Displacement of the fan beam plane and thus variations in the detector signals may be predicted and corrected. In U.S. Pat. No. 4,991,189, issued Feb. 5, 1991, assigned to the same assignee as the present invention, and incorporated by reference, a control system using a movable collimator adjusts the z-axis position of the fan beam plane as deduced from a pair of special detector cells. The special detector cells provide information to a computer model of the system which in turn is used to control the collimator and to correct the placement of the fan beam plane.
U.S. Pat. No. 4,559,639, issued Dec. 17, 1985 and assigned to the same assignee as the present invention, and also incorporated by reference, describes such special detector cells suitable for use in the above described z-axis correction. In one embodiment, shown in FIG. 4A of that patent, a single detector cell is covered with a wedge shaped opaque mask. Z-axis movement of the fan beam along this detector generates a z-signal whose intensity is dependent on that displacement. This z-signal is divided by the signal from an uncovered cell to normalize the z-signal's value to a range between one and zero. Thus, the relative displacement of the fan beam over the surface of the detectors may be determined. The normalized signal indicates that the fan beam is centered on the mask when it is equal to 1/2.
There are a number of drawbacks to the above method of detecting the z-axis position of the fan beam plane. The first is that the normalization process of dividing the z-signal by the signal from an uncovered cell requires an arithmetic division operation which is problematic in the context of a real time feedback system. A second drawback is that both detector cells, that producing the z-signal and the uncovered cell, may exhibit significant offsets in their intensity signals, that is, a finite intensity signal may be present even in the absence of any radiation. Such offsets are termed "dark currents" and operate to shift the relative center indicated by the z-signal from the actual center of the detector. For example, with dark currents, a normalized z-signal of 1/2 will not correspond to the center of the detector.
Yet a further problem with the disclosed method of producing a z-signal is that of intercell sensitivity variation, i.e., the z-axis sensitivity of each detector cell is generally different from that of its neighbors. Hence the use of a reference cell to normalize the Z signal is only partially successful.
Finally, a center value of 1/2 is inconvenient for closed loop control where a center value of zero is to be preferred.
In a second embodiment shown in the above referenced patent, the shape of the radiation receiving face of a detector cell is altered from a rectangular outline to a trapezoidal outline by slanting the dividing wall between a pair of adjacent detector cells. In this configuration, the intensity signals from the two detector cells are opposite functions of each other. The intensity signal from one detector cell increases with z-axis motion of the fan beam in one direction while the intensity signal from the other detector cell decreases. Subtracting these two signals successfully eliminates the effect of dark currents; however, the difference signal is still normalized, in this case by dividing it by the sum of the two signals. Thus, the problematic division operation is still required.
A second drawback to this embodiment is that physics and manufacturing requirements prevent sloping the dividing wall between adjacent detector cells so as to create a truly triangular radiation receiving face, but rather requires the creation of a trapezoidal receiving face. For an ionization-type detector, the dividing walls must remain electrically isolated necessitating a significant wall spacing. For solid state detectors, any deviation from the rectangular shape employed by the majority of the other detector elements is prohibitively expensive. As will be described below, it is believed that the trapezoidal receiving face adversely accentuates the effect of intercell sensitivity variation in the computation of z-axis displacement.